The isolation of new and critical human UDP-Glucuronosyltransferases has been pursued by using transferase-specific cDNAs as well as by using a 50-nucleotide conserved sequence. We have isolated two human liver cDNAs, HUG-Br1 and HUG-Br2, which encode proteins of 533 and 534 amino acids respectively, responsible for glucuronidating the 1XalphaC8 and 1XalphaC12 isomers of bilirubin to the respective mono- and to the diconjugate. The two encoded proteins have an identical carboxy terminus after amino acid residue 287, but are only 66% similar in the amino terminus. The overall similarity is 82%. The HUG-Br1 encoded message (2.6 kb) is high abundant, and that of HUG-Br2 (also 2.6 kb) is low abundant in human liver. The HUG-Br2 message is regulated by phenobarbital in at least one Old World monkey. Both messages, at 2.6 kb-length, are present in the explanted liver of a Type I Crigler-Najjar patient, but the level of that for HUG-BR1 is reduced several fold. The human liver transferase cDNA, UDPGT h-2, encoding the activity responsible for the conjugation of the critical bile acid, hyodeoxycholic acid, in humans has been identified. Previously, this isoform was shown to conjugate the 3, 4 catechol estrogens and estriols. We show also that a highly homologous form encoded by UDPGT h-1 has 1% the activity for the same three substrates. Both versions are induced in monkey by phenobarbital treatment. The significance of these results is that we can now isolate and characterize the normal human bilirubin transferase gene(s) and also that from the genome of the Type I Crigler-Najjar patient in order to determine its defect(s). The results should provide a DNA agent and critical information concerning possible gene therapy in the appropriate animal model or therapy otherwise for defective bilirubin transferase involved in several human syndromes.